Due to slight differences in the individual cylinders of an internal combustion engine, they generate slightly different torques and emissions during the combustion process. These torque differences cause the so-called “shaking” of the engine, for instance, as well as audible torque fluctuations. To compensate for such torque differences, a so-called smooth-running control, which determines and corrects the injection quantity of the individual cylinders as a function of the recorded engine speed, is known from the related art. However, this smooth-running control can be utilized only at low engine speeds since production-related tooth-pitch errors of the pulse-generator wheel normally utilized to measure the rotational speed and the crankshaft torsion interfere with the rpm measurement. The effect of these interferences is greater at high engine speeds than at low speeds. To compensate for such interference, a quantity compensation control is implemented, which takes these interferences into account with the aid of a pulse-generator adaptation and a torsion compensation. However, this quantity-compensation control, too, can be utilized only at low and medium engine speeds.
A lambda-based cylinder-compensation control is known from European Published Patent Application No. 1 215 388. Here, the lambda value of the exhaust gas of the individual cylinders is selectively equalized with the aid of a lambda-based cylinder-compensation control. To this end, correction quantities for the injection quantities of the individual cylinders are determined from the signal of at least one lambda probe. If the resolution of the lambda-probe signal is of sufficient quality, the cylinder-compensation control can be utilized in a broad engine speed and load range.
While the smooth-running control and the cylinder-compensation control do use the same control intervention, they nevertheless are competing methods as far as the purpose of the cylinder-compensation regulation is concerned, so that both methods may not be active simultaneously in an uncoordinated manner. This applies especially when cylinder-specific efficiencies, rpm-measuring errors, torque pick-offs in an engine frequency, different oxygen charging of the cylinders and different exhaust-gas recirculation rates are present.
As a consequence, the present invention is based on the objective of providing a method for controlling an internal combustion engine of the type described in the introduction, such method allowing the simultaneous intervention of both a smooth-running control and a lambda-based cylinder-control.